Zhen Dexiu 真德秀 (1178-1235), courtesy name Zhen Jingyuan 真景元 or Zhen Jingxi 真景希, style Xishan xiansheng 西山先生, was a Southern Song period 南宋 (1127-1279) scholar and philosopher. He came from Jianzhou 建州 (modern Andao 安道, Fujian) and earned the jinshi degree (see examinations) in 1199. He was appointed imperial diarist (qiju sheren 起居舍人) and Vice Minister in the Court of Imperial Sacrifices (taichang shaoqing 太常少卿) and took over the offices of prefect (zhizhou 知州) of Quanzhou 泉州 and then of Fuzhou 福州 before he became drafter in the Imperial Secretariat (zhongshu sheren 中書舍人), Vice Minister of Rites (libu shilang 禮部侍郎) and then Minister of Revenue (hubu shangshu 戶部尚書). Later on he was Hanlin academician (Hanlin xueshi 翰林學士, see Hanlin Academy) and finally Vice Counsellor (can zhizhengshi 參知政事). Zhen Dexiu is often mentioned side by side with Wei Liaoweng 魏了翁, both together called "Master Xishan and Master Heshan 西山鶴山".
As a young man Zhen Dexiu was a disciple of Zhan Tiren 詹體仁 who belonged to the school of the great Neo-Confucian philosopher Zhu Xi 朱熹. Zhen therefore belonged to the Neo-Confucian tradition of the "School of the Heavenly principle" (lixue 理學) but also integrated some thoughts of Lu Jiuyuan's 陸九淵 "School of the Mind" (xinxue 心學) in his philosophy. He said that moral behaviour (dexing 德行) was interited (de 得) from Heaven in the shape of the five Confucian cardinal virtues kindheartedness (ren 仁), appropriate behaviour (yi 義), etiquette (li 禮), wisdom (zhi 智) and trustworthiness (xin 信). The good celestial principle was transmitted not only to man but to all petty matters and minuscule things (zhi wei zhi wu 至微之物). In contrast to small things, the greatness of the human heart corresponded to the vastness of Heaven and Earth. Zhen Dexiu stressed the attachment of the Heavenly principle to physical matter, yet on the other side was of the opinion that the five virtues existed before all things and objects came into being. Self-cultivation, he said, is necessary to thoroughly investigate the Heavenly principle (qiong li 窮理) and to nourish one's mental tranquility (yang xin 養心).
Zhen Dexiu is probably the most important late Song philosopher of the School of the Heavenly Principle. Like Zhu Xi, he did not develop new theories, but was crucial for the transmission of Zhu Xi's teachings and contributed to the integration of this master into political thinking and government theory in spite of the temporary prohibition of Zhu Xi's writings of under counsellor Han Tuozhou 韓侂胄.
Zhen Dexiu has written the books Daxue yanyi 大學衍義, Xinjing 心經, Zhengjing 政經, Du Shu ji 讀書記 and Wenzhang zhengzong 文章正宗. His collected writings are assembled in the Zhen Xishan quanji 真西山全集.